In integrated semiconductor memories which enable a multiple writing and reading operation, the increasing miniaturization of the individual circuits leads to an intensified influence of production-dictated parameter fluctuations on the signal processing. Thus, by way of example, changes with regard to the length or width of individual memory cells or transistors can lead to an alteration of the weak electrical signal present on a bit line during a read-out operation. At the same time, the need for higher processing speeds in semiconductor memories continues to rise.
The rising requirements may make it necessary, inter alia, for the individual steps which are required for a read-out operation or a writing operation, for example, to be precisely coordinated with one another temporally.
If too little time has elapsed between the operation of evaluating the signals on the bit lines of a sense amplifier and the transmission of the evaluated signals onto the data lines, fluctuations in the threshold voltage of a transistor of the sense amplifier can generate an incorrect datum on a data line.
In order to avoid faults of this type, the time period between the evaluation operation and the transmission onto the data line can be increased. However, this leads to a reduction of the speed during a reading operation and thus to slower access times to the memory.
Other faults within a semiconductor memory, for example defective memory cells or bit or word lines having an increased resistance, short circuit or high parasitic capacitance, can be corrected by redundancy circuits. In this case, by way of example, complete bit lines including the memory cells are replaced by redundant elements. However, the latter require additional space on the semiconductor memory, such that the available storage capacity per unit area is reduced again.
Although the parameter fluctuations are reduced by improved and more precise production methods, nevertheless the requirements made of the processing speed and the storage capacity in semiconductor memories simultaneously increase as well. Therefore, there is the need to be able to correct production-dictated parameter fluctuations without reducing the signal processing speed or the area required.